1. Field of the Invention
This invention relates to a composition, articles and methods employing such composition. More specifically, this invention is directed toward polymeric photoconductive compositions suitable for use as charge carrier transport matrices for electrophotographic imaging members. These polymeric compositions can be modified by the incorporation therein of certain activator molecules capable of formation off charge transfer complexes with the bulky planar substituents of the polymer thus rendering the composition photoresponsive within the visible region of the electromagnetic spectrum.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging member in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface having a polarity of charge opposite to the polarity of charge on the toner particles and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g. zinc oxide/film forming resinous binder) where the photoconductive imaging layer is also an integral part of the finished copy.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above plain paper copying system, the materials used in the photoconductive insulating layer should preferably be capable of rapid switching from insulating to conductive to insulating state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulating state prior to the succeeding charging sequence will result in (an increase in the rate of dark decay) a decrease in the maximum charge acceptance of the photoconductor. This phenomenon, commonly referred to in the art as "fatigue" has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to anthracene, other organic photoconductive materials, most notably, poly(N-vinylcarbazole) have been the focus of increasing interest in electrophotography. Most organic photoconductive materials, including the polyvinylcarbazoles, lack the inherent photosensitivity to be competitive with selenium. This need for enhancement of the photoresponse characteristics of these photoconductive materials has thus led to the formulation of these organic materials with other compounds, commonly referred to as "activators". Polyvinylcarbazoles, for example, when sensitized with 2,4,7-trinitro-9-fluorenone exhibit good photoresponse and discharge characteristics and (depending upon the polarity of the surface charge) low dark decay; U.S. Pat. No. 3,484,237. Other organic resins, traditionally considered nonphotoconductive, can also be sensitized with certain activators such as Lewis acids, thus, forming charge transfer complexes which are photoresponsive in the visible band of the electromagnetic spectrum, see for example, U.S. Pat. Nos. 3,408,181 - 190.
Photoconductive polymers, such as poly(N-vinylcarbazole) have also been used in combination with other photoconductive pigments in the formation of electrophotographic imaging members; see U.K. Patent No. 1,343,671-- photoconductive pigments dispersed within a poly(N-vinylcarbazole) matrix and U.K. Patent No. 1,337,228--a photoconductive layer laminated to a layer of poly(N-vinylcarbazole). In both of the above systems, the poly(N-vinylcarbazole) does not participate in photogeneration of charge carriers since the source of illumination in projection of image information onto the imaging member is substantially outside the range of spectral response of vinylcarbazoles. The poly(N-vinylcarbazole) merely acts to transport the charge carriers generated by the other photoconductive materials and, in fact, inadvertent photogeneration of charge carriers by the carbazole matrix can impair the efficiency of transport and result in incomplete discharge of the sensitizing charge in the illuminated areas. Although these composite systems have distinct advantages over the organic photoconductors described previously, (both with regard to electrophotographic speed and mechanical strength), the unsensitized carbazole polymer is moderately chemically unstable. Because of this relative instability, the carbazole will react with oxygen in the air and ozone produced during sensitization and consequently its electronic properties will gradually decline.
Accordingly, the object of the invention is to remove the above as well as related deficiencies in the prior art.
More specifically, it is the principal object of this invention to provide a polymeric composition suitable for use as a charge carrier transport matrix.
It is another object of this invention to provide a polymeric photoconductive composition which is chemically stable to the extent that it does not undergo progressive deterioration of its electronic properties in an electrophotographic environment.
It is yet another object of this invention to provide a polymeric composition which can be sensitized with activators and thus form a charge transfer complex suitable for use in electrophotographic imaging members and methods.